This invention relates to a control system which controls the locking and unlocking of a differential gear mechanism.
It is well known to provide vehicles, such as farm tractors, with differentials which can be selectively locked or unlocked. In a simple form, differential lock control systems are in use wherein a solenoid-operated differential lock control valve is controlled by a single, manually-operated floor switch. However, with such a system, the vehicle operator must continuously depress the floor switch in order to keep the differential locked. On both row crop and four-wheel drive (4WD) tractors with differential locks, a locked differential can interfere with the steering of the tractor. One solution to this problem, usually applied to row crop tractors, has been to unlock the differential in response to a left or right wheel brake application since the brakes are often used to assist the turning of a row crop tractor. Such a system is described in U.S. Pat. No. 2,874,790 (Hennessey).
Another solution to this problem has been to control the locking of the differential as a function of the relative angle between mechanical components of the tractor steering system. An example of this is also shown in the Hennessey patent. Another example of a steering angle differential lock control system may be found in co-pending U.S. Patent Application, Ser. No. 503,969, filed June 13, 1983, and assigned to the assignee of the present invention. However, in the Hennessey system, the differential can be both locked and unlocked by operation of a steering angle sensitive switch. In the system described in Application, Ser. No. 503,969, both the brake and steering-responsive switches are operable to unlock, but not to lock, the differential. Accordingly, it would be desirable to have a differential lock control system for a tractor wherein brake application is operable only to unlock the differential and wherein a steering-responsive device is operable only to re-lock the differential. Furthermore, it would be desirable to control the unlocking and re-locking of the differential as a function of additional vehicle operational conditions, such as wheel slip, vehicle speed and draft force.